H2—O2(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H2—O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells (a.k.a. SPE (solid polymer electrolyte) fuel cells) are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O2), or air (i.e., O2 mixed with N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
During the conversion of the anode and cathode reactants into electrical energy, the fuel cell, regardless of the type, produces anode and cathode effluents that are exhausted from the fuel cell stack. Water (also known as product water) is generated at the cathode electrode based on electric-chemical reactions between hydrogen and oxygen occurring within the MEA. Efficient operation of the fuel cell stack depends on the ability to provide proper and effective water management in the system.
During operation of the fuel cell stack, the cathode reactant is typically supplied with an excess amount (stoichiometric amount larger than 1.0). The excess cathode reactant is used due to the oxygen partial pressure in the flow fields of the fuel cell stack decreasing as the reactions occur throughout the fuel cell stack. Another reason for supplying excess cathode reactant is to assist in the removal of liquid water from the cathode side of the fuel cell stack. While the performance of the fuel cell stack benefits from the higher stoichiometric quantity of cathode reactant, a lower stoichiometric quantity will be favorable for an efficiency point of view due to the necessity of supplying power to the airmover (a parasitic device) to supply the cathode reactant. Thus, it would be advantageous to develop an operation strategy which satisfies the needs (product water removal and stable electricity production) of a fuel cell stack but maintains a power demand placed upon the air machinery low.